linux-kernel - Re: [PATCH 1/2] KVM: X86: Move ignore

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Date:   Thu, 25 Jun 2020 09:25:40 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Peter Xu <peterx@...hat.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, Vitaly Kuznetsov <vkuznets@...hat.com>
Subject: Re: [PATCH 1/2] KVM: X86: Move ignore_msrs handling upper the stack

On Thu, Jun 25, 2020 at 10:09:13AM +0200, Paolo Bonzini wrote:
> On 25/06/20 08:15, Sean Christopherson wrote:
> > IMO, kvm_cpuid() is simply buggy.  If KVM attempts to access a non-existent
> > MSR then it darn well should warn.
> > 
> > diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> > index 8a294f9747aa..7ef7283011d6 100644
> > --- a/arch/x86/kvm/cpuid.c
> > +++ b/arch/x86/kvm/cpuid.c
> > @@ -1013,7 +1013,8 @@ bool kvm_cpuid(struct kvm_vcpu *vcpu, u32 *eax, u32 *ebx,
> >                 *ebx = entry->ebx;
> >                 *ecx = entry->ecx;
> >                 *edx = entry->edx;
> > -               if (function == 7 && index == 0) {
> > +               if (function == 7 && index == 0 && (*ebx | (F(RTM) | F(HLE))) &&
> > +                   (vcpu->arch.arch_capabilities & ARCH_CAP_TSX_CTRL_MSR)) {
> >                         u64 data;
> >                         if (!__kvm_get_msr(vcpu, MSR_IA32_TSX_CTRL, &data, true) &&
> >                             (data & TSX_CTRL_CPUID_CLEAR))
> > 
> 
> That works too, but I disagree that warning is the correct behavior
> here.  It certainly should warn as long as kvm_get_msr blindly returns
> zero.  However, for a guest it's fine to access a potentially
> non-existent MSR if you're ready to trap the #GP, and the point of this
> series is to let cpuid.c or any other KVM code do the same.

I get the "what" of the change, and even the "why" to some extent, but I
dislike the idea of supporting/encouraging blind reads/writes to MSRs.
Blind writes are just asking for problems, and suppressing warnings on reads
is almost guaranteed to be suppressing a KVM bug.

Case in point, looking at the TSX thing again, I actually think the fix
should be:

diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index 5eb618dbf211..64322446e590 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -1013,9 +1013,9 @@ bool kvm_cpuid(struct kvm_vcpu *vcpu, u32 *eax, u32 *ebx,
                *ebx = entry->ebx;
                *ecx = entry->ecx;
                *edx = entry->edx;
-               if (function == 7 && index == 0) {
+               if (function == 7 && index == 0 && (*ebx | (F(RTM) | F(HLE))) {
                        u64 data;
-                       if (!__kvm_get_msr(vcpu, MSR_IA32_TSX_CTRL, &data, true) &&
+                       if (!kvm_get_msr(vcpu, MSR_IA32_TSX_CTRL, &data) &&
                            (data & TSX_CTRL_CPUID_CLEAR))
                                *ebx &= ~(F(RTM) | F(HLE));
                }

On VMX, MSR_IA32_TSX_CTRL will be added to the so called shared MSR array
regardless of whether or not it is being advertised to userspace (this is
a bug in its own right).  Using the host_initiated variant means KVM will
incorrectly bypass VMX's ARCH_CAP_TSX_CTRL_MSR check, i.e. incorrectly
clear the bits if userspace is being weird and stuffed MSR_IA32_TSX_CTRL
without advertising it to the guest.

In short, the whole MSR_IA32_TSX_CTRL implementation seems messy and this
is just papering over that mess.  The correct fix is to invoke setup_msrs()
on writes to MSR_IA32_ARCH_CAPABILITIES, filtering MSR_IA32_TSX_CTRL out of
shared MSRs when it's not advertised, and change kvm_cpuid() to use the
unpriveleged variant.

TSC_CTRL aside, if we insist on pointing a gun at our foot at some point,
this should be a dedicated flavor of MSR access, e.g. msr_data.kvm_initiated,
so that it at least requires intentionally loading the gun.